This invention relates to an apparatus for use with a tubing string used in conducting drill stem tests of oil and gas wells. More particularly, the apparatus relates to a check valve apparatus allowing trapped fluid to flow from the interior of the test string to the well annulus when the testing string is being lowered in a well bore into sealing engagement with a wireline set production type packer.
During the drilling of oil and gas wells, drilling fluid known as mud is used, among other things, to maintain formation fluids in intersected formations by virtue of its hydrostatic pressure. In order to allow the formation fluids to flow to the surface for analysis, it is necessary to isolate the formation to be tested from the hydrostatic pressure of the drilling fluid in the well annulus. This is done by lowering a tubular string to the formation to be tested, and then sealing the well annulus between the tubular string and above the formation with a packer.
Typically a tester valve is included at the lower end of the tubular string and is lowered in the closed condition such that a lower pressure exists in the center bore of the tubular string. After the formation is isolated from the well annulus, the tester valve is opened to lower the pressure in the well bore adjacent the formation to be tested such that formation fluids may flow from the formation into the lower end of the tubular string and from there to the surface.
Pressure sensors are typically included in the test string such that the tester valve may be opened and closed and pressure recordings made to evaluate the production potential of the formation being tested.
Two types of packers may be used. The first type is a packer which may be incorporated in a tubular string and expanding by manipulation of the tubing string to effect the seal between the walls of the well bore and the tubular testing string. A second type is a wireline set production packer which is lowered and attached to the walls of the well bore at the desired location. The tubular string having a seal assembly at its lower end, is then lowered into the well bore until the seal assembly is seated in the production type packer to effect the seal necessary to isolate the formation.
It will be understood that if a production type packer is used, fluid trapped in the well bore below the production packer will be compressed as the tubular string is further lowered into place after the seal assembly has effected its seal in the production packer. This trapped fluid in the well bore below the packer must be displaced back into the formation as the seal assembly is further lowered into the packer. The displacement of drilling fluid into the formation is undesirable in that it may seal the port spaces in the formation through which oil and gas must be produced. Also, if an annulus pressure operated well tester valve having a pressure operated isolation valve such as that disclosed in U.S. Pat. No. 3,964,544 or U.S. Pat. No. 3,976,136 is used, the compression of fluid in the central bore of the well string below the tester valve will increase the operating pressure of the tester valve to an undesirably high level.
The use of the disclosed embodiments prevents high pressure from the trapped fluid from developing which might otherwise damage the packer, the pressure recorder, the tester valve, or other tools in the testing string. Also, this trapped fluid might support the testing string and prevent its downward movement to completely seat in a hanger. When a test valve in the testing string is subsequently opened, the trapped fluid will be released allowing the testing string to fall which may in turn damage the tubing of the string, the hanger or the packer.
In the disclosed embodiments of the present invention, a check valve means is provided below the tester valve and above the seal assembly at the lower end of the testing string, and is designed to allow compressed fluid in the central bore of the testing string below the closed tester valve to escape to the well annulus above the packer. When the well annulus pressure is increased to operate tester valves such as those disclosed in the aforementioned U.S. Pat. Nos. 3,964,544 and 3,976,136, the check valve prevents pressure from increasing in the testing string central bore, and a blocking mechanism is activated to block the check valve means in a closed position. The blocking means is then locked in the closed position such that treating operations of the formation, as disclosed in U.S. Pat. No. 3,976,136, may be conducted wherein specialized chemicals, such as an acid, may be displaced into the formation without escaping into the well annulus through the check valve.
The invention disclosed makes the use of annulus pressure operated testing apparatus in combination with a production type packer more efficient in that the pressure level necessary to operate the testing tools is not unduly raised, and the operation of the tools is not otherwise affected.